A conventional pneumatic tool disclosed in U.S. Patent Application Publication 20140231485 includes a trigger, a force transmission element, a first control valve, a second control valve, a control piston that is associated with the first control valve, and a locking piston that is associated with the second control valve and that is movable within a housing cap. The second control valve is activated to drive movement of the locking piston upon depression of the trigger. The first control valve is activated to drive movement of the control piston when both of the trigger and the force transmission element are depressed. A fastener can be fired by the conventional pneumatic tool by continuously depressing the trigger and subsequently depressing the force transmission element. After the trigger is continuously depressed by a predetermined time period, the locking piston is moved onto a moving path of the control piston upon the activation of the second control valve, so as to prevent the movement of the control piston. As such, when a time delay between the depression of the trigger and the depression of the force transmission element is less than the predetermined time period, the first control valve is activated to drive movement of the control piston upon the depression of the force transmission element, so as to fire the fastener. When a time delay between the depression of the trigger and the depression of the force transmission element is greater than the predetermined time period, the control piston cannot be moved for firing the fastener upon the activation of the first control valve by virtue of the depression of the force transmission element since the control piston is locked by the locking piston.
However, since the locking piston serves as a latch for preventing the movement of the control piston, the locking piston and the control piston may easily be worn, and the air-tightness between the locking piston and the housing cap may be affected by the control piston.